Stepper class processor utilisation improvements

At version 0.1.1, the design of the stepper class makes use of two interupts, one which is a "controller" interrupt; the TIMER overflow interrupt, and the other interrupt is used to count the number of pulses, the TIMER Compare interrupt.
Both of these interrupts will be triggered for each STEP pulse generated, which essentially doubles the utilisation of the CPU, which can get quite a lot with faster pulses.

The aim of this issue is to reduce the interrupt utilisation, such that either:
a) Only the counter interrupt is needed, with the overflow called when a change is needed (i.e. new speed, or position achieved)
or
b) Only one interrupt is required, which handles everything